The present invention relates to a printing device utilizing a continuous-form recording sheet on which a printing operation is to be executed by a so-called electrophotographic system. The system has a transfer charger retraction mechanism for retracting a transfer charger used in the electrophotographic system from an operating position thereof, when the printing operation is not executed. This specifically relates to a printing device capable of definitely separating the continuous-form recording sheet that has been pressed to a photoconductive drum by means of the transfer charger, from the photoconductive drum by retraction of the transfer charger.
A conventionally known image forming apparatus, such as a printer, makes use of the electrophotographic system, by which a latent image is formed by exposing a photoconductive agent on a surface of a photoconductive drum which is arranged to be rotated in the presence of light. The latent image is developed by adhering toner thereto. The toner is transferred onto a recording sheet arranged to be synchronously fed with the rotation of the photoconductive drum. Finally, the transferred toner is fixed by means of a fixing unit. As an image forming apparatus, there is an electrophotographic printer for printing information on a folding-type continuous sheet with feed holes, which is called a fan-folded sheet that is similar to that used in a conventional line printer and the like.
The fan-folded sheet is arranged in such a manner that a boundary of each of the pages thereof is provided with a perforated tear line for cutting, and thus, the fan-folded sheet is alternately folded along the perforated tear line in the opposite direction and delivered to the image forming apparatus in a stacked state.
Incidentally, a printer using the continuous-form sheet as a recording medium must begin the printing operation at a position such that each of the pages is separated apart from the perforated tear line for a predetermined distance. That is, the printing operation is started at a position separated from a leading edge of the sheet in a direction along which the printing operations are executed. In the above arrangement of the electrophotographic system, an exposure unit, a development unit and a transfer unit are disposed around the photoconductive drum and put into operation sequentially as the photoconductive drum is rotated. Thus, an image is transferred when the exposed portion of the photoconductive drum gets to a transfer position during the rotation of the photoconductive drum. Therefore, a print start point on the continuous-form sheet must be adjusted by relatively moving, i.e., rotating, the photoconductive drum with respect to the continuous-form sheet. In other words, before the printing operation is executed, it is necessary to control a positional relationship between the image forming start position on the photoconductive drum and the continuous-form sheet, in such a manner that the position of the photoconductive drum, at which exposure is to be started, coincides with the print start position of the page of continuous-form sheet.
If the continuous-form sheet is kept in contact with the photoconductive drum, i.e., in a transfer state, during the above-described position adjusting operation, a problem arises in that the photoconductive agent on the surface of the photoconductive drum is scratched or worn shortening the life of the photoconductive drum. Alternately, the continuous-form sheet is stained with toner remaining on the surface of the photoconductive drum. To cope with this problem, there have been prior art systems for retracting the transfer charger from the operating position, at which the transfer charger causes the continuous-form sheet to contact the photoconductive drum, disclosed in, for example, Japanese Patent Provisional Publication HEI 2-103076. In such conventional charger retracting systems, the transfer charger is designed to be retracted upon the end of a feeding operation of the continuous-form sheet.
Nevertheless, a problem arises in that even if the transfer charger is retracted from the operating position after completion of the printing operation, the continuous-form sheet, having been synchronously fed with a rotation of the photoconductive drum at a predetermined speed, cannot be instantly stopped. Then, the continuous-form sheet is fed slightly further by the inertia thereof. Thus, the portion of the continuous-form sheet to be contacted with the photoconductive drum is loosened, because the continuous-form sheet is fed by a feeding mechanism located at a downstream side of the photoconductive drum. Therefore, even if the transfer charger is retracted, the continuous-form sheet is contacted with the photoconductive drum, resulting in the continuous-form sheet being stained by the toner when the printing is resumed. This problem is more often caused when the folding direction of the perforated tear line is projected toward the photoconductive drum side.